The current estimated world human population is 6,350,538,084.There is no complete database on the world's population, and humans are constantly being born (at the rate of about 3 per second) and dying. However, it is clear that the world's population continues to grow, and at rates that are unprecedented prior to the 20th century. By some estimates, there are now one billion young people in the world between the ages of 15 and 24.

When was six billion reached?
The United Nations Population Fund designated October 12, 1999 as the approximate day on which world population reached six billion. This was about 12 years after world population reached five billion, in 1987.

Rate of population increase:
The last 70 years of the 20th century saw the biggest increase in the world's population in human history. The following table shows when each billion milestone was met:

1 billion reached in 1802.
2 billion reached in 1927.
3 billion reached in 1961.
4 billion reached in 1971.
5 billion reached in 1987.
6 billion reached in 1999.

From the figures above, the world's population tripled in 72 years, and doubled in 38.
The UN estimated in 2000 that the world's population was then growing at the rate of 1.2 percent (or 77 million people) per year. This represents a decrease in the growth rate in 1990, mostly due to decreased birth rates.

Every year the world’s population gains nearly 90 million people, as of 2004.

Forecast of world population: The future growth of population is difficult to predict. Birth rates are declining slightly on average, but vary greatly between developed countries (where birth rates are often at or below replacement levels) and developing countries. Death rates can change unexpectedly due to disease, wars and catastrophes, or advances in medicine. The UN itself has issued multiple projections of future world population, based on different assumptions.

The current projections from the UN's Population Division, from their 2002 World Population Prospects database [1], are:

Year Population (billions)
2010 6.8
2020 7.5
2030 8.1
2040 8.6
2050 8.9